1. Field of the Invention
The present invention relates to an electronic apparatus for detecting a remaining battery capacity, and a method for controlling the electronic apparatus.
2. Description of the Related Art
In the related art, a technique for detecting a remaining battery capacity is disclosed in, for example, Japanese Patent Laid-Open No. 2002-298932. A device disclosed in Japanese Patent Laid-Open No. 2002-298932 includes a temperature detector for detecting a voltage, discharge current, and temperature of a battery, and a storage unit for storing a charge capacity of the battery that is fully charged. The device further includes a current integrating unit for integrating the detected discharge current, and a calculation unit for calculating a remaining capacity of the battery. The calculation unit first determines a discharge efficiency for discharge characteristics of the battery on the basis of the detected temperature, current, and voltage (power) (see FIG. 7).
The calculation unit then subtracts the integrated current from the product of the stored charge capacity and the discharge efficiency to calculate a remaining capacity, and obtains the resulting value as the remaining capacity of the battery in the current temperature and discharge power conditions. That is, in the remaining-battery-capacity detection technique of the related art, when the discharge efficiency changes with battery power and temperature, a remaining battery capacity is calculated using the current power and temperature and is displayed.
However, as shown in FIG. 2, cameras such as digital cameras have a difference on the order of several times to dozens of times between power P0 determined when the cameras are started and a remaining battery capacity is detected before the start of a photographing operation (a period from time T0 to time T1 shown in FIG. 2) and maximum power PM1 determined during the photographing operation (a period from time T3 to time T6 shown in FIG. 2). Such existing cameras having a large difference between the power determined during a photographing operation and the power determined before the start of the photographing operation have experienced the following problems.
A first problem is as follows. As described above, in the related art, when the discharge efficiency changes with battery power and temperature, a remaining battery capacity is calculated using the current power and temperature and is indicated to a user. Thus, before a photographing operation is started, a remaining battery capacity is detected and the user is notified that the discharge efficiency is high and the remaining capacity is sufficient. During the photographing operation, however, the power increases and the discharge efficiency decreases, resulting in the possibility of discharge failure.
This problem will now be specifically described with respect to examples shown in FIGS. 2 and 7. In the period from time T0 to time T1 shown in FIG. 2 (before the start of a photographing operation), a remaining battery capacity is detected and the power P0 is indicated to the user as the remaining capacity. Since the discharge efficiency at the power value P0 for a temperature of −20° C. is 97%, it is determined that the discharge efficiency is high and the remaining capacity is sufficient. In the period from time T3 to time T6 shown in FIG. 2 (during the photographing operation), however, the power increases and the discharge efficiency decreases. In particular, at the maximum power PM1 for a period from time T4 to time T5, the discharge efficiency for a temperature of −20° C. is 72%, resulting in the possibility of discharge failure.
For example, it is assumed that the temperature is −20° C., the charge capacity of the fully charged battery is 2000 mAh, the integrated discharge capacity is 400 mAh, the power P0 is 1 W, and the maximum power PM1 is 12 W. In this case, referring to FIG. 7, the discharge efficiency at a power of 1 W is 97%, and the remaining capacity is given as:
                              Remaining          ⁢                                          ⁢          capacity          ⁢                                          ⁢                      (                          1              ⁢                                                          ⁢              W                        )                          =                  (                                    2000              ×                              97                /                100                                      -            400                    )                                        =                  1540          ⁢                                          ⁢          mAh                    
Referring to FIG. 7, further, the discharge efficiency at a power of 12 W is 72%, and the remaining capacity is given as:
                              Remaining          ⁢                                          ⁢          capacity          ⁢                                          ⁢                      (                          12              ⁢                                                          ⁢              W                        )                          =                  (                                    2000              ×                              72                /                100                                      -            400                    )                                        =                  1040          ⁢                                          ⁢          mAh                    
Another problem occurs with an arrangement in which a minimum remaining capacity due to changes in temperature or power in use is stored and a remaining battery capacity is displayed based on the minimum remaining capacity.
Even though it is determined that there is no remaining capacity in a battery check with the power determined during a photographing operation, if the same battery is attached, there arises a problem in that due to a battery check under the condition of lower power, it is erroneously determined that battery life seems to be left. Further, if the communication interval between remaining capacity checks is long, a time lag is caused between the storage of the minimum remaining capacity and the reading thereof by the apparatus, and it is difficult to detect a remaining capacity at the maximum power point. This does not provide an accurate indication of remaining capacity.